Cultured mouse fibroblasts which are transformed by RNA viruses, a DNA virus or a chemical agent, all secrete a 35,000 M polypeptide (major excreted protein, MEP) in large amounts. Nontransformed murine fibroblasts secrete this protein in much lower amounts. We have purified this protein and prepared antisera against it. The protein, whose biologic function is unknown, undergoes extensive modification prior to secretion. We are studying this system as a model of regulation of protein synthesis, processing and secretion as it is affected by transformation and agents which mimic the transformed state, such as tumor promoters.